This line, "ye the marriage of two minds admit," is spoken after Romeo sees Juliet for the first time.
In essence, Romeo is questioning whether true love can exist, or if he's ever truly felt it before. He's comparing Juliet to a jewel on a dark surface, implying she's a shining beacon of beauty against the darkness. He's also comparing her to a beautiful dove amongst a flock of crows, further highlighting her uniqueness.
He's captivated by her beauty and intrigued by the prospect of love, implying that he's never truly experienced it before. It's a powerful moment that sets the stage for their forbidden love story.